1. Field of the invention
The invention relates to polymer particles, e.g. solid (SPS) or expandable polystyrene (EPS) particles. More particularly, the invention relates to a process for preparing polymer particles that optionally contain a water-retaining agent, to polymer particles per se, to a method for incorporating water into the polymer particles after the particles are formed and/or after the particles are formed into an article, e.g. boards, panels, and sheets, and to foamed articles made from the polymer particles.
2. Background Art
Particles that contain a polymer of a vinylarene monomer and a blowing agent are generally known as expandable polymer particles. A well-known type of expandable polymer particle is expandable polystyrene. Expandable polystyrene may be produced on a commercial scale by suspension polymerization. The blowing agent is generally a low-boiling hydrocarbon, such as a C3-C8 hydrocarbon, in particular pentane isomers. The expandable polystyrene is used for making foamed articles that are produced by expanding the polystyrene particles. In the expansion process, the hydrocarbon blowing agent is released and may be emitted into the environment. Such emissions are regarded as being undesirable and ways are sought to avoid such emissions. One way is to recover or burn the emitted hydrocarbon. Another way is to reduce the amount of hydrocarbon blowing agent in the expandable polymer particles.
In U.S. Pat. No. 5,096,931, expandable polystyrene particles are described which contain polystyrene, a small amount of a polar polymer that acts as a water retaining agent, water, and a reduced amount of hydrocarbon blowing agent. Although the amount of hydrocarbon foaming agent has been reduced, such foaming or blowing agent must still be present in order to achieve satisfactory expansion of the particles.
GB-A-1106143 discloses a process for preparing water expandable polystyrene particles by mixing via vigorous mechanical agitation, styrene monomer, water, and an emulsifier with a free-radical initiator to obtain an emulsion containing small droplets of water. Subsequently, the emulsion is suspended in an aqueous phase and the resulting suspension is subjected to polymerization. In order to achieve a satisfactory expansion certain amounts of organic blowing agents are included.
Other processes for preparing water expandable styrene polymers (WEPS) are disclosed in U.S. Pat. Nos. 6,160,027; 6,127,439; 6,242,540, and 6,528,570. The process of U.S. Pat. No. 6,160,027 partially polymerizes a mass of styrene incorporated with water together with an emulsifier, and then polymerizes the partially polymerized mass to a high conversion using conventional suspension techniques.
The process of U.S. Pat. No. 6,127,439 produces expandable polymer of a vinyl aromatic monomer and a polar water absorbing monomer. The monomers are partially polymerized and then suspended in water so that the resulting beads contain 3 to 40 weight percent water that acts as the sole or predominant blowing agent.
U.S. Pat. No. 6,242,540 discloses particles of a copolymer of a vinylarene and a copolymerizable compound containing a polar moiety and a vinyl moiety containing water. The particles are prepared by forming a mixture of monomers and small amounts of water. This mass is polymerized under agitation to 20% to 70% conversion and then suspended in water for completing the polymerization. The resulting polymer beads contain finely dispersed water and are useful as an environmentally acceptable blowing agent.
U.S. Pat. No. 6,528,570 discloses polymer particles containing a polymer of a vinylarene monomer, a porous silicate compound, and 1.5% by weight of water, based on the weight of vinylarene, water and porous silicate compound. The porous silicate compound is selected from alumino silicates and amorphous silica particles.
A further example of a process for preparing expandable styrene polymers, which comprise water as the sole blowing agent is disclosed in U.S. Pat. No. 6,387,968. In this process, the styrene is polymerized in aqueous suspension in the presence of 0.1 to 15% by weight of a solid substance, for example, carbon black or graphite insoluble in water and in styrene.
In general, a disadvantage of the above processes for preparing water expandable styrene polymers is that the process is carried out in two stages, the first stage being a pre-polymerization stage where the emulsifiers or water retaining agents are first emulsified or blended into the styrene/polystyrene or polymer mixture, and the second stage being suspension polymerization where the mixture is polymerized in suspension.
U.S. Pat. No. 6,342,540 (BASF) discloses a simpler, single-stage process for preparing WEPS. This is achieved by mixing a melt of a styrene polymer with from 3 to 20% by weight of water and with an emulsifying agent under super atmospheric pressure, extruding the melt into a cooling medium maintained under super atmospheric pressure, and pelletizing the extrudate after cooling. In this instance, the water is incorporated into the polymer in the extrusion process at the same time as the emulsifying agent.
Solid polymer particles are also well known to those skilled in the art. Such particles generally are those that do not contain a blowing agent when the particles are produced. However, a blowing agent may be incorporated into the polymer during further processing, such as through an extrusion process for forming foamed boards. Examples of solid polymer particles, e.g. polystyrene particles, include those taught in U.S. Pat. Nos. 6,166,099; 6,274,641; 6,420,444; 6,476,149; 6,433,092 and 6,608,141. These polymer particles are formed into foamed boards or uni-axially or bi-axially oriented sheets.
There is still a need in the art to provide a process for preparing water expandable polystyrene (WEPS) particles in a simpler, single-stage process.
There is also a need in the art to provide expandable polymer particles that do not have water added and/or contained in the polymer until the polymer is ready for expansion, which may involve preparing articles, such as panels or sheets from these polymer particles in various conventional processing lines and operations.